1. Field of the Invention
The present invention relates to an adaptive predistortor type distortion compensator which operates an adaptive algorithm so that errors of a reference signal and a feedback signal outputted from an amplifier may be minimized in order to calculate a distortion compensation coefficient and increase the efficiency of the amplifier by compensating the nonlinear distortion of the amplifier using this distortion compensation coefficient.
2. Description of the Related Art
FIG. 1 shows an example of the basic configuration of a conventional adaptive predistortor type distortion compensator. An input baseband signal (1) is a complex baseband signal comprising I signal and Q signal.
The input baseband signal (1) is multiplied by a distortion compensation coefficient in the multiplier 18 and is inputted to the quadrature modulator 19. A signal modulated by the quadrature modulator 19 is converted into an analog signal by the digital-to-analog converter 20. This analog signal is multiplied by a periodic wave of the local oscillator 21 in the multiplier 22, and is converted into a signal of an intermediate frequency (IF). The signal of the intermediate frequency generated in this way passes through the band-pass filter 23; is multiplied by the periodic wave of the local oscillator 24 in the multiplier 25; is converted into a radio frequency (RF); is amplified and transmitted by the (power) amplifier 26.
The input baseband signal (1) is converted into a power value by the power computing unit 11, and is used to index a distortion compensation coefficient from the distortion compensation table 10 which stores the distortion compensation coefficient in a memory using the power value of the input baseband signal (1) as an address.
The input baseband signal (1) is also inputted as a reference signal (3) to the subtractor 16 which obtains an error signal e (t) from a feedback signal (2) and the input baseband signal.
The output from the amplifier 26 is separated by the feedback path 32 and is attenuated by the attenuator 27. The output is multiplied by the periodic wave of the local oscillator 28 in the multiplier 29 and is returned to the intermediate frequency. Also, the output is converted into a digital signal in the analog-to-digital converter 30; is demodulated in the quadrature demodulator 31; and I signal and Q signal are generated. I signal and Q signal pass through the low-pass filter 17, and are inputted to the rotator 13, and at the same time are also inputted to the subtractor 16 as a feedback signal (2).
An error signal e(t) which is a difference between a reference signal (3) and a signal in which a feedback signal (2) from the amplifier 26 is attenuated to a reasonable amplitude by the attenuator 27 (which is shown by ATT in FIG. 1) is computed as a digital signal. In the example shown in FIG. 1, a clipped LMS algorithm is used as an adaptive algorithm, and the distortion compensation coefficient hn(p) (p is the power of the reference signal) is updated for the quantitized amplitudes of the signals using e(t) by the following expression.hn(p)=hn−1(p)+μe(t)det[hn−1(p)]det[y(t)*], wherein
            det      ⁢                          [              R        +        jI            ]        =                  1                  2                    ⁡              [                              sgn            ⁢                                                  ⁢                          (              R              )                                +                      j            ⁢                                                  ⁢                          sgn              ⁡                              (                I                )                                                    ]                        sgn      ⁡              (        x        )              =          {                                    1                                              (                              x                ≥                0                            )                                                                          -              1                                                          (                              x                <                0                            )                                          
Note, however, that y(t) indicates a feedback signal; * indicates a complex conjugate; j indicates an imaginary unit; and p indicates an update step of a distortion compensation coefficient. This computing is conducted by the rotator 13, update storage unit 12, multiplier 15, adder 14 and distortion compensation table 10 shown in FIG. 1. This algorithm is the one in which a circuit configuration is simplified by multiplying the part to be multiplied by an error signal in which complex multiplication is required in an ordinary LMS algorithm by det [hn−1(p)] det [y(t)*] i.e. by conducting only the phase rotation of 0, π/2, π and 3π/2.
Refer to Kokai (Jpn. unexamined patent publication) No. 9-68733 for the details of an adaptive predistortor type distortion compensator.
If the signal level of the reference signal (3) or the feedback signal (2) becomes 0 (zero) or only a noise component, various functions of the distortion compensator do not properly operate.